1. Field of the Invention
The present invention relates to a process for fabricating a vertical-cavity surface-emitting semiconductor laser, which is useful as an optical source for optical interconnection that optically connects chips or boards to each other or for conducting two-dimensional parallel signal processing, and to a vertical-cavity surface-emitting semiconductor laser.
2. Description of the Related Art
Vertical-cavity surface-emitting semiconductor lasers, which are easy to construct a two-dimensional array and enables high efficient coupling with fibers without use of lenses for coupling because of their illumination pattern being circular, are considered important as an optical source for optical interconnection or two-dimensional parallel signal processing and also important for the purpose of reducing power consumption because they permit extreme lowering of threshold current by means of an ultrafine-cavity structure.
FIG. 1 is a cross-sectional view showing a conventional vertical-cavity surface-emitting semiconductor laser along the direction vertical to a crystal face thereof (cf. (1) B.-S. Yoo, H. Y. Chu, H.-H. Park, H. G. Lee and J. Lee, IEEE Journal of Quantum Electronics, vol. 33, No. 10, 1997, pp. 1794-1800; and (2) C. Chang-Hasnain, Y. A., Wu, G. S. Li, G. Hasnain, K. D. Choquette, C. Caneau and L. T. Florez, Applied Physics Letters, vol. 63, No. 10, 1993, pp. 1307-1309). The laser of FIG. 1 comprises devices including a p-GaAs substrate 101, which has thereon in order a p-AlyGa1-yAs/AlzGa1-zAs (0<y<z) distributed Bragg reflector (DBR) mirror 102 (the dashed portion showing AlzGa1-zAs and the white portion showing AlyGa1-yAs), a non-doped AlwGa1-wAs lower spacer layer 103, a GaAs/AlxGa1-xAs (x≦w) multiple quantum well active layer 104, a non-doped AlwGa1-wAs upper spacer layer 105, an n-AlyGa1-yAs/AlzGa1-zAs (0<y<z) DBR mirror 106, a semiconductor buried layer 107, a lower electrode 108, an insulator 109, an upper electrode 110, and an element separating structure 111. The respective layers of DBR are set to a thickness corresponding to one fourth of the quotient obtained by dividing the lasing wavelength by the refractive index of each layer.
Among the devices shown in FIG. 1, an AlGaAs/AlGaAs n-i-p-i structure or an amorphous GaAs layer has been reported as the buried layer 107, each exhibiting single transverse mode lasing operation. However, the above-described structures do not achieve optical constriction due to refractive index optical waveguide but are of an anti-guide waveguide structure. Therefore, in principle, a plurality of transverse modes can occur but not a single transverse mode. In this case, higher order transverse modes are cut off to achieve a single transverse mode operation by utilizing the outer portion of the waveguide having a higher loss. However, in dynamic characteristics in which the carrier density inside the active layer varies widely, there arises the problem that an unstable operation occurs, that is, higher order modes may emerge depending on the carrier density distribution in the active layer.